1. Field of the Invention
The invention relates to a vehicle having a wheel assembly with an in-wheel motor that reduces the rate of output of the in-wheel motor using a reduction mechanism, which includes a counter gear mechanism and a planetary gear set, and drives a wheel.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-A-2004-168211, for example, describes a related wheel assembly that reduces the rate of output of an in-wheel motor using a reduction mechanism, which includes a counter gear mechanism and a planetary gear set, and drives a wheel, as well as a vehicle having this wheel assembly.
In this kind of a vehicle having a wheel assembly with an in-wheel motor, the gears of the counter gear mechanism and the planetary gear set normally have a helix angle to prevent noise (i.e., helical gears are normally used instead of spur gears). In this case, the directionality of the helix angle of the helical gears of the counter gear mechanism and the planetary gear set is opposite in the left and right wheels so the gears used in the left wheel are different from the gears used in the right wheel (i.e., the gears used in the left and right wheels are not identical).
On the other hand, unlike the reduction mechanism of the transmission which is used by (i.e., common to) both the left and right wheels in a normal vehicle, there is a need to make parts in the reduction mechanisms of the in-wheel motors in a vehicle having wheel assemblies with in-wheel motors identical because the motors that are used in the left and right driving wheels are independent from each other. In particular, as in the vehicle having a wheel assembly with an in-wheel motor described above, there is an especially large number of gear parts when a two or more step reduction mechanism is used, which further increases the need for identical parts in the left and right wheels (i.e., parts in the left wheel that are the same as parts in the right wheel).
One way to do this is simply to make the helical gears in the counter gear mechanisms and the planetary gear sets in the left and right wheels identical by making the directionality of the helix angle of the helical gears of the counter gear mechanisms and the planetary gear sets the same.
However, this angle of torsion of helical gears generates axial force (i.e., thrust force). As a result, the bearings must have specifications that can satisfy loads in different directions in the left and right wheels, which is not easy to achieve. In particular, if the directionality of the helix angle of the helical gears of the counter gear mechanism that is responsible for a first reduction is the same for the left and right wheels, it is difficult to structure both bearings that support the rotor of the motor from the vehicle inside and outside satisfy the requirements relating to both bearing life and maximum allowable rotation speed. This is because on one hand it is necessary to increase the bearing diameter in order to extend the bearing life, while on the other hand it is necessary to reduce the bearing diameter in order to increase the maximum allowable rotation speed. Obviously, these two necessities are in contradiction with one another. For example, in order to ensure the bearing life, if the axial force generated by the helix angle of the helical gears on the left wheel side is in the direction toward the vehicle inside, it is necessary to increase the diameter of the bearing on the side that receives that axial force (e.g., the bearing on the vehicle inside). On the other hand, on the right wheel side in this case, the axial force generated by the helix angle of the helical gears is in the direction toward the vehicle outside so it is necessary to increase the diameter of the bearing on the side that receives that axial force (e.g., the bearing on the vehicle outside). In this way, if the necessary bearing life is to be ensured, the diameter of both bearings that support the rotor of the motor must ultimately be increased which reduces the maximum allowable rotation speed. That is, it is no longer possible to obtain a well-designed bearing that is suitable for the motor side.